1. Field of the Invention
The present invention relates in general to the process of microphotolithography in which a photosensitive layer and an anti-reflective coating are involved for forming structural patterns on semiconductor substrates.
2. Description of Prior Art
Photoresist for microphotolithography is a thin coating that undergoes chemical reactions when exposed to ultraviolet radiations. The chemical reactions cause the coating more or less soluble in developers. If a photoresist film is exposed to radiation image of a photomask, bias of dissolution rate between exposed and unexposed areas incurs. The image of photomask is transferred to the photoresist film after development.
If exposure to ultraviolet radiations increases solubility, the photoresist is said having a positive tone in the field of invention. As illustrated in FIG. 1, radiated regions of positive-tone photoresist are removed by developer while unexposed regions are retained on substrate. Positive-tone photoresist is widely used in the field of photolithograph for integrated circuit (IC) fabrication. Currently, the most popular photoresist is based on de-protection mechanism.
De-protection mechanism requires a resin with phenolic or carboxylic groups on side chains. The phenolic or carboxylic groups are protected by labile groups in initial films. Initial films are therefore insoluble in alkaline developers. Upon exposure to ultraviolet radiations, labile groups are hydrolyzed, and phenolic or carboxylic groups are de-protected. The phenolic or carboxylic groups make the radiated films soluble in developers. The hydrolyzation is catalyzed by photo-generated acid. One of the widely used groups is tertiary butoxycarbonyl (t-BOC).
If exposure to ultraviolet radiations decreases solubility, the photoresist is said having a negative tone in the field of invention. As illustrated in FIG. 1, radiated regions of negative-tone photoresist are retained on substrate while unexposed regions are removed by developer. Negative-tone photoresist is widely used in processes of fabricating micro-electro-mechanical systems (MEMS) or packaging integrated-circuit chips. It is hard to find instances of using negative-tone photoresist in delineating high-resolution images of precursor structures for integrated-circuit fabrication.
The less popularity is attributed to lack of viable chemistry platforms for negative photoresist. Reported negative photoresists are based on the intuitive mechanism of crosslinking. Radiations cause film resin to crosslink under catalyzation of photoacid generators. Un-crosslinked films require organic solvents to remove. However, crosslinked films are prone to swelling in organic solvents. Swelling films are not suitable for high-resolution delineating.
Obviously, photoresist of prior art can only respond to radiations in one mode, either positive tone or negative tone. For any photoresist to respond to radiations in both ways simultaneously is against conventional wisdom and beyond imagination before disclosures of the present invention. The present invention materializes dual-tone technology the first time. As shown in FIG. 1, the dual-tone films are capable of splitting projected images. Resolution is two times higher than photoresist of prior art. The films are consisted of silicon as high as 40%. Etch performance is equivalent to hardmask films.